(1) Field of the Invention
The invention relates to a power supply, and more particularly to a switching power supply capable of preventing an AC voltage and an AC current from coupling to a secondary side of a transformer.
(2) Description of the Prior Art
An electronic device usually utilizes a battery to serve as a power source so that it can be conveniently carried. However, the time in use is limited due to the problem of the power storing capacity of the battery. The problem of the limited time always exists even if the efficiency of the battery is continuously improved and enhanced.
When the electronic device is continuously used for a very long period of time, the battery is replaced with an AC-to-DC power device (e.g., a power supply) in order to solve the problem of charging the battery. The most economic implementation of this power supply is an linear power supply having a transformer composed of silicon steel sheets. Although the linear power supply can solve the problem of the short time in continuous using of the electronic device, the size and the weight of the linear power supply are greater than those of the switching power supply, and the poor efficiency of the linear power supply cannot be accepted by the user because the working frequency of the linear power supply is very low. To meet the trends of the high efficiency, the light weight, the thin thickness, the short length and the small size, the cheaper linear power supply is ultimately replaced by the switching power supply even in the occasion of the low power application.
FIG. 1 shows the most frequent implementation of a switching power supply. When an input voltage is at a positive half cycle, the current flows through an input filtering device 201 and a bridge rectifier 202 to charge a filtering capacitor 203, and then flows back to the power source from a grounding terminal of the filtering capacitor 203. When a main switch 208 is switched to turn on, a transformer 205 transforms the charges stored in the filtering capacitor 203 to form an output voltage according to a voltage ratio, and an output rectifying diode 206 and a filtering capacitor 207 respectively rectify and filter the output voltage to provide a DC voltage for output. When the input voltage is at a negative half cycle, the current flows through the input filtering device 201 and the bridge rectifier 202 to charge the filtering capacitor 203, and then flows back to the power source from the filtering capacitor 203. This circuit operates stably according to the rule.
On the other hand, the size of the switching power supply is usually very small in the high-frequency trend, and the sandwich winding method is usually adopted due to the consideration of the coupling effect between the windings of the transformer, so the parasitic capacitor of the transformer is usually larger. In addition, in order to satisfy the electromagnetic compatibility and electromagnetic interference, a bridging capacitor 204 has to be added between the primary side and the secondary side of the transformer. The bridging capacitor 204 in an equivalent circuit is connected in parallel to a capacitor between the parasitic capacitance.
When the input voltage is electrically connected to the filtering capacitor 203 through the input filtering device 201 and the bridge rectifier 202 by a line (L), the positive terminal of the filtering capacitor 203 has an AC voltage with a positive half cycle relative to the ground. Similarly, when the input voltage is electrically connected to the filtering capacitor 203 through the input filtering device 201 and the bridge rectifier 202 by a neutral (N), the negative terminal of the filtering capacitor 203 has an AC voltage with a negative half cycle relative to the ground. The positive half cycle and the negative half cycle are coupled to the secondary side through the bridging capacitor 204, thereby causing an AC low-frequency carrier voltage (or referred to as a common mode voltage) on the circuit of the secondary side relative to the ground. When the leakage current is being tested (the equivalent model is shown in FIG. 5), the common mode voltage of the primary side is coupled to the node of the secondary side through the bridging capacitor 204 and is then discharged to the ground through the loop formed by the testing equipment because the method and the object of the testing equipment are to simulate the behavior and the equivalent circuit of the human body. Because the common mode voltage exists at each node of the secondary side, a current (I) flows through the testing equipment to generate the leakage current. Thus, when the common mode voltage is low, the generated leakage current is low, and the danger to the human body is also low.
In addition, when this AC low-frequency carrier voltage is supplied to a load 108, which may be an analog or digital audio amplifier, a home appliance serving as an audio transfer medium (e.g., a phone or even an advanced VoIP phone), or a medical equipment, which may contact the human body, the following problems occur.
1. When the human body touches the circuit node of the secondary side, the voltage of the secondary side is coupled the ground through the human body so that a loop is formed, and the energy stored in the bridging capacitor 204 is discharged through the human body such that the human body tends to shock. This is because the human body itself is a conductive object R and the human body is at the position with the ground potential. The associated equivalent model is illustrated in FIG. 2.
2. When the load 108 is an audio product, the common mode voltage enters an analog or digital signal amplifier circuit through a DC output wire. Then, the low-frequency common mode voltage and the analog or digital signal are mixed and then enter the amplifier circuit for amplifying the mixed signal multiple times. Finally, a low-pass filter (LPF) outputs the amplified signal, and an audio signal with the low-frequency carrier is generated on the speaker such that a low-frequency AC hum is generated. The circuit model of generating the AC hum is shown in FIG. 3.
3. When the load 108 is a phone, an IP phone (VoIP), a router or a modem, the low-frequency common mode voltage is coupled to the secondary side through the bridging capacitor 204. When the load is connected, the common mode voltage enters the system with the power. When the user touches the phone apparatus, the equivalent resistance of the human body enables the common mode voltage to be grounded so that the loop may be formed. So, the microphone of the phone generates the low-frequency AC hum. The circuit model of generating the AC hum is illustrated in FIG. 4.
It is to be noted that the parasitic capacitor of the transformer is a parameter that cannot be eliminated. So, the problem of the common mode voltage coupling cannot be eliminated, and it is challenging to seek a countermeasure to reduce or suppress the AC hum.